fbdev: sh_mobile_meram: Correct pointer check for YCbCr chroma plane

The check was intended to test if we have a valid pointer to write into,
but it mistakenly checks the pointer contents instead.

Since a valid pointer is mandatory for the chroma data if a YCbCr format
is used, the pointer check has been removed.

Signed-off-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>